Theory of Atomic Structure and Processes

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The unifying theme of this proposal is the development of new measurement tools through the combined application of both high***precision theory and experiment to atoms. In brief, the theoretical side rests on a foundation of our essentially exact nonrelativistic***wave functions for atoms such as helium and lithium, combined with the state-of-the-art for relativistic and quantum electrodynamic***effects. A comparison with experiment then provides a window into new physical phenomena. A prime example from the past grant period***is the use of the isotope shift in atomic transition frequencies to deduce the size and shape of exotic "halo" nuclei such as helium-6***and helium-8. This work at the interface between atomic and nuclear physics is done in collaboration with the top experimental groups***in the world at high energy particle accelerators such as Argonne (U.S.), GSI (Germany) and TRIUMF (Canada). The accelerator-based***projects are subjected to rigorous standards of feasibility and significance before being approved.***Future work will build on our past experience and extend it in several directions. First, the isotope shift method developed by***us is now well recognized as the most accurate method, and in many cases the only method available, to measure the size of***halo nuclei. These exotic structures have extra planetary neutrons surrounding a tightly bound core of neutrons and protons. The***significance of the results is that they enable one to distinguish among the various theoretical models proposed for the effective***forces holding the nucleus together. By studying structures that fall apart easily, we learn about the forces holding them together.***Our previous work on the halo nuclei of helium, lithium and beryllium will next be extended to boron, in collaboration with experimental groups at GSI, Germany. Fascinating trends are emerging across the periodic table of isotopes.***Second, we are opening up a new area of study with applications to search for new physics beyond the standard model of elementary***particle physics. When a halo nucleus decays via the weak interaction, it emits both a beta-particle (high energy electron) and a neutrino. The standard model predicts the angle between the two outgoing particles, and any deviation from it would signal new physics beyond the standard model. We are currently calculating the recoil contribution due to the atomic electrons that must be taken into account in interpreting the measurements. Our preliminary results indicate that there is a serious disagreement with a landmark 1963 experiment. There is an urgent need for both new experimental work and further calculations that take into account the atomic electrons that are "shaken off" following the beta decay process. This work is an ongoing collaboration with an experimental group at the University of Washington.***Other proposed work focuses on fundamental tests of atomic theory and quantum electrodynamics.**